Electronic design automation (EDA) software systems usually include a circuit design simulator (e.g., design simulator), such as Simulation Program with Integrated Circuit Emphasis (SPICE) or a variant, for solving and verifying circuit designs. Generally, to simulate operation of a circuit design, the design simulator parses and operates on a circuit netlist (also known simply as a netlist) that describes a circuit model of the circuit design and specifies conditions under which operation of the circuit model will be simulated. The circuit model may include model elements (e.g., transistors), parameters (e.g., process/device), and connectivity (e.g., topology), while the conditions may include inputs to the circuit model for the simulation intervals (e.g., waveform profiles).
As part of the simulation, a circuit design simulator may perform transient analysis to determine output signals of a circuit design in response to input signals. An aspect of the transient analysis includes determining the impedance state of a node of the circuit design. The impedance state of a particular node may be in high-impedance (highz) state or in low-impedance (lowz) state. A particular node may be considered in high-impedance state when there is no conducting path from the particular node to ground or a voltage source (e.g., direct current [DC] power supply), and may be considered low-impedance state when there is a conducting path from the particular node to ground or a voltage source.
Unfortunately, in highz state, a particular node of a circuit design can cause a transistor in the circuit design to switch unexpectedly, which in turn can cause an integrated circuit (IC) implemented by the circuit design to fail once on silicon. As a result, detection of nodes in highz state (hereafter, highz nodes) in a circuit design is important early in the design cycle.